Tape storage technology is routinely used for routine system back up and long-term data archiving. Various tape technologies such as DAT, DLT, and 8 mm, have been developed over recent years to meet industry needs for higher performance and increased data storage capacity. While they differ widely in format, capacity, and performance, conventional tape storage devices rely on some form of track-following architecture. Whether the underlying recording technology is based on linear serpentine or helical scan recording, track-following architecture tape drives attempt to read a track completely in a single pass using a single head assembly and operating at a single fixed tape speed. The drive mechanism and media tolerances are tightly controlled to maintain a very precise alignment between the path traced by the heads and the written tracks on a tape.
Track-following architecture tape drives save some common inherent limitations. To either write or read data, track-following tape technologies depend upon a constant head-to-tape speed for linear recording or constant track pitch for helical scan recording. Accordingly, the tape drive must either receive stream data at a constant transfer rate, or, when this does not occur, initiate a stop/backhitch/start sequence.
Due to normal speed variations found in network and workstation applications, the data transfer rate to or from the host rarely matches the tape drive's fixed read or write speed. Whenever a mismatch occurs, the read or write operation is suspended and the tape is repositioned backwards to allow enough space to accelerate again to the forward operating speed. The time required for this repositioning cycle, known as backhitching, increases data retrieval time.
Furthermore, backhitching not only impairs performance, but also seriously impacts data reliability. In operation, backhitching induces extremely high transient forces that greatly increase tape wear and reduce the mechanical reliability of the drive.
Another limitation imposed by the track-following architecture is the significant area of the tape required to record track-following servo data. This servo overhead consumes physical space on the tape and limits the amount of data that can be written onto an individual tape.
Finally, the tight tolerances required for track-following technologies challenge the production margins of even the most sophisticated manufacturing facilities. Minute differences in assembly result in data availability and tape interchange issues. Nominal production variations, sometimes accentuated by differences in handling or environmental conditions, can make it impossible to exchange media between supposedly identical units from the same manufacturer. In other words, data written on one tape drive may not be read on another similar tape drive.
In view of the inherent limitations of track-following tape control architectures and need in the tape drive industry to keep pace with the disk drive industry in terms of performance, capacity, and cost, a need exists for a new methodology for delivering the required performance and capacity while reducing costs to the end-user.